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Question: Replacing rear 9-leaf spring with the monoleaf


Well-known member
Feb 25, 2002
Cleveland, Ohio
ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
For years I've been kicking around replacing my rear 9-leaf with the single fiberglass or composite spring. I'm now to that point since I'm doing so other minor rear-end work.
I did some searching on the forums and read through a few threads, but none of the quite got me satisfied yet.
I 100% believe the overall quality of the ride will be better. So no need to point that out.

My driving style is spirited around town, maybe the rare red-light race here or there, and occasional trips to the drag strip for shits & grins.
I've had my current spring so long that I don't really have a comparison for "harsh" versus "soft" ride. It's just always been what it is. I'd probably rate it more harsh than soft.
My current spring initially had the car riding high, so it already has the extended length bolts to even out the right height.

1) Last time I was investigating, the proverbial "everyone" said to get the heaviest weight spring. Here are some of varying weight... CLICK!
Is this still the general consensus? Does my driving style(s) referenced above change that?

2) Technology changes over time. Looks like there are fiberglass springs and composite springs. As it goes, the composite spring carries a notable price premium. Is there a significant difference? What does the additional price get you?

Are there any other questions I should be asking or any information I've left out?

Oh, BTW... Hello again, old friends! :w
I broke a main leaf in my '65 once and replaced the stock 9 leaf spring with a 7 leaf F41 spring out of a '66 427 car. It rode considerably stiffer and it also improved the cornering handling with no other changes. It's a trade off for sure. You can improve handling and lessen squat on acceleration with a stiffer spring but your butt will not be as happy.

One benefit of going to a composit is that you will shed weight off the rear of the car. The only experience I have with a composit spring is that the '81 came with one. If I autocrossed or ran twisty roads in a spirited manner I know I would benefit from a stiffer composit spring. But I don't drive on the edge very often. I'd say that if you want to upgrade handling you should probably go with a stiffer composit and reap the benefits of the weight loss too.

I haven't changed mine out,either.
IMO,I would rather try to find an OEM spring for my '81 rather than an aftermarket one.
Going to a composite rear spring in a C3 is a good thing as far as weight goes. They weigh a lot less than a steel spring. They also have better durability than do steel spring.

The problem with them comes with ride. Because a steel leaf spring has inherent damping characteristics due to the friction of the leaves moving against each other as the spring flexes, when you change to a single plastic leaf spring, you eliminate that damping. As a result some feel that the rear suspension of a C2/C3 with a retrofitted composite spring is under damped, particularly in cases where a ride movement in the rear is rapid and uses a lot of the available travel.

The solution is more aggressive rear shock absorbers.

I have a Vette Brakes and Products composite spring on my '71 and it's been there for more than 30 years. I also have Bilstein shocks with sport valving.
I went with a 330lb/in spring on mine and I sometimes wonder if i should have gone for a stiffer spring. Vansteel do a 400lb/in spring. (they recommend it for racing but whats the fun is only using it for that?)
As others have said you will want to make sure you have appropriate shocks for this change.
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Thanks to everyone who's replied thus far. I'll go with the higher lbs/in spring.
One of the main questions I was asking though, is there there are apparently pure fiberglass springs and there are composite springs. In my original post, the link to takes you to a product page that seems to indicate there are pure fiberglass springs and composite springs. I was attempting to learn the difference between the two. (That was question #2 of my original post)

I just went back and re-read the product page. I don't think there's a difference. I think the manufacturer, Hyperco, simply calls their spring a "composite" spring, whereas the general(?) spring that is offered is simply referred to as "fiberglass".

Due to available funds, it looks like I'm gonna be going with the 355lb spring from Corvette Central. Ecklers has the 440lb spring, but it's a bit more expensive and doesn't appear to come with new hardware (bolts, bumpers, etc.) I don't drag race all that often and I don't autocross, so I'm gonna hope the 355lb doesn't negatively impact what I'm already used to. If so, I suppose like Hib and others have said, I can go to a more aggressive shock.
This whole spring replacement is mostly just vanity at this point, so I'm not overly concerned. :L
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leave the 9 leaf alone .......... Maintain the ride height :happyanim:
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So the rear leaf spring replacement is complete. Ride height is unchanged. The car is now parked for the winter, but initial feedback from the few drives I was able to squeak in...
  • Ride quality seems about the same. Maybe a bit more soft / less harsh?
  • The rear end seems like it's ever so slightly more inclined to jump out on me under added throttle through a corner.
    • What might be the cause of that?
      • The one obvious thing I haven't checked is my tire pressures. I might be running a bit high in the rears.
      • I also might be driving a bit more agressively since I also had severely worn u-joints replaced at the same time.
The car has gotten "loose" under power out of turns because you greatly increased the rear spring rate without changing anything in the front.

You'd have been better to select a rear spring rate closer to what you had before or...change springs and bars to a package that works well together.
My '81 is a 4spd and came the factory with a metal leaf spring, even though the automatic transmission cars were equipped with a fiberglass spring. The spring in my car was making a racket, so in 1987 (or so) I replaced it with a fiberglass spring. The ride high stayed the same. The ride was better. And it was quiet. All these years later I have never had a problem with it.

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